1. Field of the Invention
The invention is directed to a process for producing a device for direct thermoelectric energy conversion whereby the efficiency of energy conversion from heat to electricity, or vice versa, is substantially increased and is directed to a composition of matter to be used in the manufacture of devices for direct thermoelectric energy conversion.
2. Description of the Prior Art
Using the powder metallurgy technique as a way of producing the composition of matter, as defined above, careful attention must be paid to a recent development that took place at the National Institute of Standards and Technology-NIST. The new technology development program, or invention, titled: “Synthesis of Fine-Powder Polycrystalline Bi—Se—Te, Bi—Sb—Te, and Bi—Sb—Se—Te Alloys for Thermoelectric Applications” was reported by J. Terry Lynch in the June 1996 issue of the International Thermoelectric Society: “Thermoelectric News”. Precursors to alloys having the general compositions of matter: Bi—Se—Te, Bi—Sb—Te and Bi—Sb—Se—Te are synthesized by aqueous co-precipitation and metal-organo complex methods. Hydrogen reduction of the precursors produced the alloys in fine-powder, polycrystalline form. The method is simpler than conventional melt-processing and produced an 88–92% yield in laboratory-scale tests. The new method reduces equipment, materials and labor costs, by producing fine powders directly, thus eliminating the crushing and sieving steps necessary after melt-processing. Precursor synthesis occurs at under 100 Celsius in aqueous solution from commonly available chemicals. Alloy synthesis at 300–400 Celsius, lower than melt-processing temperatures, yields more than 88% product compared with theory. Scale-up to continuous production is possible using common chemical flow reactor technology. This new development or invention improves the efficiency and cost-effectiveness of producing solid-state thermoelectric cooling and refrigerating devices. Therefore, it is very likely worthwhile to investigate this new development still further, with the objective of adapting or extending it to the compositions of matter, which constitute the basic embodiments of the present invention. This would substantially eliminate one basic drawback of the powder-metallurgy technique, specifically unwanted contamination, or doping, of the composition of matter, namely with iron, Fe, coming from the steel grinding balls and the steel vessels of the planetary ball mill. That is because a planetary ball mill will not be used, since crushing and pulverization of the composition of matter, or alloy, will no longer be needed. Furthermore, this new technique developed at NIST, if successfully adapted to the compositions of matter, herein specified and claimed, will also help overcome or eliminate the main disadvantages associated with the melt metallurgical techniques previously described. These are the need to agitate or vibrate the constituents during melting, in order to assure the production of a homogeneous alloy, as well as the requirement of maintaining the molten ingredients in an atmosphere of argon or helium, while subjecting them to a relative pressure of between 2 and 30 physical atmospheres, in order to suppress the loss of magnesium, and thus ensure obtaining a stoichiometric alloy.
Thermoelectricity, or thermoelectrics, as it is nowadays called, owes its existence to the discovery by Thomas Johann Seebeck of the first thermoelectric effect, in 1821, ever since known as the Seebeck effect, or Seebeck coefficient. In 1833, Peltier discovered the second thermoelectric effect, ever since known as the Peltier effect. Seebeck discovered that a compass needle would be deflected, when placed near a closed loop, made of two dissimilar metals, when one of the two junctions was kept at a higher temperature than the other. This establishes the fact that a voltage difference exists or is generated, whenever there is a temperature difference between the two junctions. That would also depend on the nature of the metals involved. Peltier found that temperature changes occur, accompanied by the absorption or rejection of heat, at a junction of dissimilar metals, whenever an electrical current is caused to flow through the junction. In 1838, Lenz came forth with the explanation that heat is either absorbed or released at a junction depending on the direction of current flow. Furthermore, Sir William Thomson, later known as Lord Kelvin, who, along with German physicist Rudolf Julius Emmanuel Clausius, became famous around the middle of the nineteenth century for their formulation of the first and second laws of thermodynamics, as well as for their discovery and establishment of the concept of entropy, also made important contributions to thermoelectricity. He discovered a third thermoelectric effect: The Thomson effect, which relates to the heating or cooling of a single homogeneous conductor subjected to a temperature gradient. He also established four important equations, correlating all three effects, namely the Seebeck, Peltier and Thomson coefficients. These are known in the art as the Kelvin relations and are found in any standard textbook on thermoelectricity, or direct energy conversion. Thermoelectricity, moreover, received a major boost in 1885, when Lord Rayleigh considered or suggested using the Seebeck effect for the generation of electricity. A milestone in our general understanding of thermoelectricity, specifically, how to best use and apply it for the direct conversion of heat into electricity, or vice versa, was brought about in 1911 by Altenkirch. He created a satisfactory theory of thermoelectricity for power generation and cooling. He reasoned that, for best performance, the Seebeck coefficient, or thermoelectric power, as it is currently called, must be as high as possible, likewise the electrical conductivity must be as high as possible, while the thermal conductivity should be as low as possible. Thus, we have the power factor: PF=S2σ=S2/ρ, where S=Seebeck coefficient or thermoelectric power, σ=electrical conductivity and ρ=electrical resistivity, which quantity, that is the power factor, must be increased as much as possible, or maximized, and k=thermal conductivity, which must be decreased as much as possible, or minimized. Thus, Altenkirch was led to establishing the following equation:
  Z  =                              S          2                ⁢        σ            k        =                            S          2                          ρ          ⁢                                          ⁢          k                    =              PF        k            where Z is known as the thermoelectric figure of merit, and has the dimensions of K−1. This equation can be rendered dimensionless, by multiplying it by some absolute temperature, T, which could be that of the hot junction of the thermoelectric device. This gives rise to another quantity: The dimensionless thermoelectric figure of merit, ZT, which, like, Z can also be used in the evaluation of the performance, and energy conversion efficiency, of any thermoelectric material or device.
The modern period in thermoelectrics actually started when the attention of engineers and scientists focused more and more on semiconductors. The latter are defined as those substances or materials whose electrical conductivity is intermediate between that of metals and that of insulators. Comparison was being made of so-called minerals, which is the way semiconductors were known, or called, at that time, versus metals. It was found that metals had the advantage of malleability, relatively constant properties, i.e. practically independent of temperature, as well as chemical stability, whereas minerals or semiconductors, when moderately, or even heavily, doped, possessed a relatively high Seebeck coefficient, S, and consequently a moderate thermoelectric figure of merit, Z. Disadvantages of metals were found to be their low Seebeck coefficient, S, their low thermoelectric figure of merit, Z, and the limit imposed by the Wiedemann-Franz law on the ratio between thermal conductivity, which is mainly electronic, and electrical conductivity. This law specifies that such a ratio, when plotted versus the absolute temperature, T, represents a straight line, or linear relationship, for metals, whose slope is the Lorenz number, L. So the Wiedemann-Franz law for metals may be expressed as follows:
      k    σ    =                    k        el            σ        =    LT  
where kel=electronic thermal conductivity.
For metals k=kel=total thermal conductivity, since the lattice thermal conductivity is insignificant, or negligible.
Disadvantages of minerals, or semiconductors, were their brittleness, temperature dependent properties and lack of chemical stability. As a matter of fact, the dependency of the properties of semiconductors on temperature makes all theoretical analyses in respect of their performance, figure of merit, energy conversion efficiency, coefficient of performance, power generated, or consumed, heat absorbed or rejected at the cold junction, heat rejected, absorbed or transferred at the hot junction, when used as thermoelectric materials, or thermoelements, much more complicated than those for metals. Thus, metals were found to be more useful as thermocouple wires, whereas semiconductors were deemed more appropriate for the manufacture of small modules, constituting the basic thermoelements, legs or branches of thermoelectric devices. It should be emphasized that many of the technological difficulties encountered in thermoelectricity emanate from the fact that thermoelectric devices comprise modules, or thermoelements, made of semiconductors, which generally do not posses the flexibility, resilience and chemical stability of metals.
Further progress in thermoelectricity was made in the 1930s, when synthetic or compound semiconductors were studied for the first time. In 1947, Maria Telkes developed and constructed a thermoelectric power generator with a 5% energy conversion efficiency. Moreover, in 1949, A. F. Ioffe established the theory of semiconductor thermoelectricity. He wrote the two pioneering books: “Physics of Semiconductors,” and “Semiconductor Thermoelements and Thermoelectric Cooling.” Semiconductors are actually substances or materials having an electrical conductivity that is intermediate between that of metals and that of insulators. An increase in the electrical conductivity of semiconductors can normally be achieved by increasing the number of free charge carriers therein. This can be done by introducing the atoms of a suitable foreign element, compound or material, generally known as the doping agent, or impurity, in an appropriate amount, or proportion, into the semiconductor. The latter process of incorporating the atoms of a foreign element or impurity into a semiconductor is called doping. Thus, doping is carried out in such a way as to bring about a free charge carrier concentration in the semiconductor of between 1×1018 and 5×1020 carriers per cubic centimeter at room temperature. Doped semiconductors with a free charge carrier concentration of the order of 1018 carriers per cm3 are considered “lightly doped”, those with a free charge carrier concentration of the order of 1019 carriers per cm3 are considered “moderately doped”, while those with a free charge carrier concentration of the order of 1020 carriers per cm3 are known as “heavily doped” semiconductors. It should be noted here that the power factor, or S2σ, is maximized at a free charge carrier concentration of about 1019 carriers per cm3. Likewise, the thermoelectric figure of merit, Z, is also maximized at about the same free charge carrier concentration of 1019 carriers per cm3. These are approximate rules of thumb that are applicable to all semiconductors in general, but may vary slightly from one semiconductor to another.
Most semiconductors are non-elemental, or synthetic, i.e. compounds, and generally have low to moderate energy band gaps. Most earlier semiconductors involved elements of higher atomic number and atomic mass. This was done intentionally, in order to select elements having a thermal conductivity as low as possible, thus optimizing the thermoelectric figure of merit. Consequently, the applicable rule here is that the higher the atomic number, and atomic mass, of an element is, the lower is its thermal conductivity. This has undoubtedly led to the: “heavy element selection criterion.” Thus an element with a high atomic mass, i.e. a heavy element, ought to be selected and given preference over other lighter elements, since it was a foregone conclusion that such an element would have the lowest possible thermal conductivity. Consequently, this would be conducive to the highest possible thermoelectric figure of merit. This type of reasoning was very prominent and proved fruitful in the thirties, forties and fifties, and was spearheaded beyond any shadow of a doubt, by A. F. Ioffe himself. It certainly initiated the research and development work that led to the establishment, to this very day, of bismuth telluride, Bi2Te3, and lead telluride, PbTe, as the two most prominent, and most frequently used, thermoelectric materials. The former has been widely used, ever since, in thermoelectric refrigeration, or cooling, while the latter has been successfully employed in both thermoelectric cooling and thermoelectric power generation. However, this notion, or concept, that the thermal conductivity of an element is lower, the higher its atomic mass or atomic number, is not necessarily true all over the Periodic Table. It is thus only partly valid. Its validity becomes more noticeable and accentuated, starting with the column representing group IVB elements, as we move downwards to lower and lower rows, and likewise as we move to the right, to group VB and VIB elements. Thus, despite its earlier successes in the thirties, forties and fifties, in the selection of good thermoelectric elements and compounds, the heavy element selection criterion or concept does not universally hold regarding all elements of the Periodic Table. Moreover, this earlier observation, concept or criterion, aside from helping identify and develop two of the best materials, thus far, in the field of thermoelectricity, it simultaneously also helped identify, or discover, a total of five, mainly heavy, elements, namely: lead, bismuth, antimony, tellurium and selenium. All these five elements, also having low thermal conductivities, were the major contributors to the successes achieved in thermoelectrics in the thirties, forties and fifties, namely in thermoelectric cooling, and thermoelectric power generation. Thus, more synthetic, or compound, semiconductors came into being, or were eventually developed, as a result of the aforementioned criterion. Examples are, just to name only a few: lead selenide, lead antimonide, lead telluride selenide, lead antimonide selenide, bismuth antimonide, bismuth selenide, antimony telluride, silver antimony telluride, bismuth telluride selenide and bismuth antimonide selenide.
Summarizing, since the electrical conductivity of a semiconductor has to be generally increased, in order to maximize the thermoelectric power factor: PF=S2σ=S2/ρ, then semiconductors are normally either moderately, or heavily, doped. Furthermore, in order to, likewise, maximize the thermoelectric figure of merit:
  Z  =            PF      k        =                                        S            2                    ⁢          σ                k            =                        S          2                          ρ          ⁢                                          ⁢          k                    the thermal conductivity must also be reduced, or lowered, as much as possible. In order to achieve this, one must apply, and make full use of, the “A. F. Ioffe Heavy Element Selection Criterion,” referred to earlier in this specification, by reviewing the Periodic Table of the Elements and considering the possibility of using the five elements, occupying the seventh or bottom row, and simultaneously belonging to Groups IVB, VB, VIB, VIIB and VIII of the Periodic Table, for that purpose. These five elements possess the highest five atomic numbers possible in the Periodic Table, namely, 100, 101, 102, 103 and 104, and the corresponding atomic masses are 257, 258, 259, 262 and 261, respectively. The corresponding names of these elements, likewise, are Fermium, Fm, Mendelevium, Md, Nobelium, No, Lawrencium, Lr, and Dubnium, Unq, respectively. These are the names recommended by the International Union of Pure and Applied Chemistry, IUPAC, and modified as suggested by the Berkeley (USA) researchers. The aforementioned five elements, having the highest atomic numbers and atomic masses in the Periodic Table, unfortunately, are not good for our purpose, that is for thermoelectric energy conversion. They are all metallic, synthetic, radioactive and short-lived, and must therefore be discarded. One must thus shift one's attention to the five elements lying immediately above the aforementioned ones, namely above Fm, Md, No, Lr and Unq, in the 6th row. Accordingly, one finds or identifies five new elements, to choose the prospective best, or ideal, thermoelectric semiconducting material from. These are lead, bismuth, polonium, astatine and radon. Radon, Rn, is a heavy gaseous radioactive element and hence must be ruled out. Astatine, At, is a highly unstable radioactive element and must also be excluded. Polonium, Po, is a naturally radioactive metallic element and must likewise be eliminated as a possible choice. That leaves only bismuth, Bi, and lead, Pb, with atomic numbers of 83 and 82, and atomic masses of 208.98 and 207.2, respectively, as our ideal semiconducting thermoelectric elements, or materials. It should have become evident to any physicist working on thermoelectrics at that time, either theoretically, or experimentally, or both, and this very probably refers to A. F. Ioffe himself, that further alloying, or reacting, of either bismuth or lead, with tellurium, which is a non-metallic semiconducting element, would produce compounds that are definitely semiconductors. Moreover, reacting or alloying each of bismuth and lead with tellurium, yielding the compounds bismuth telluride, Bi2Te3, and lead telluride, PbTe, respectively, would further reduce the thermal conductivity of the resulting compounds and bring it to some intermediate value between those of the original ingredients. Thus, alloying bismuth with tellurium, reduces the thermal conductivity of the former to some intermediate value in between that of bismuth and that of tellurium. Although lead, unlike bismuth, behaves more as a metal, rather than as a semiconductor, which must have made it relatively more difficult to be identified, or thought of, initially, as a potential thermoelectric material, yet alloying or reacting it again with tellurium has brought about another outstanding synthetic, or compound, semiconductor, with singular or unique thermoelectric properties, and that is lead telluride, PbTe. While bismuth telluride is more well known for its widespread or prevalent use in thermoelectric refrigeration, lead telluride, despite fierce competition from the silicon-germanium alloys, namely Si0 7Ge0.3, is, to this very day, one of the best materials for thermoelectric power generation. The two synthetic materials, or compound semiconductors, i.e. Bi2Te3 and PbTe, were thus beyond any shadow of a doubt responsible for the big successes and triumphs of thermoelectricity, before the advent of the sixties. In conclusion, the first thermoelectric refrigerator, or heat pump, was built in 1953, while the first thermoelectric power generator with a 5% efficiency was constructed in 1947, by Maria Telkes.
Most semiconductors have low to moderate energy band gaps. The energy band gap is the single most important factor to be considered in the development, design or synthesis of any new semiconducting material, as to its possible or potential use for direct thermoelectric energy conversion. The width of the forbidden energy band gap is crucial for thermoelectric materials, because the width of the gap is a measure of the energy required to remove an electron from a localized bond orbital and raise the electron to a conducting level. A material with a narrow energy band gap is undesirable, because this implies that the material will become degenerate or intrinsic at a relatively low temperature. According to a formula given by Pierre Aigrain, the narrower the energy band gap of a material is, the lower the temperature at which the material becomes intrinsic, or degenerate, and thus useless for thermoelectric energy conversion. The reason for the foregoing is that when a material becomes degenerate, both its electrical and thermal conductivities increase, however, its thermoelectric power, which is raised to the power 2, also decreases quite substantially, and this has a detrimental effect on the figure of merit. Again, from Aigrain's formula, it can be inferred that the wider the energy band gap of a material is, the higher will be the maximum hot junction temperature at which a device, comprising such a material, can be operated, while maintaining a high thermoelectric figure of merit. A device in which both the maximum hot junction temperature, and the thermoelectric figure of merit, are adequately high, will also have a high overall energy conversion efficiency.
On the other hand, a very wide energy band gap is still undesirable, because it implies a greater difficulty of removal of electrons form localized bond orbitals to conduction bands. Consequently, a moderately wide energy band gap, namely about 0.6 electron volt, is adequate for direct thermoelectric energy conversion. This figure was suggested by Pierre Aigrain, as one of the characteristics of good thermoelectric materials. The following table shows the energy band gaps of various semiconducting intermetallic compounds, or synthetic semiconductors, and relevant semiconducting and metallic elements.
EnergyEnergyEnergyCompoundBandCompoundBandCompoundBandor ElementGap eVor ElementGap eVor ElementGap eVCa2Si1.9PbS0.37α-LaSi20.19Ca2Sn0.9InSb0.27OsSi21.4Ca2Pb0.46InAs0.47Os2Si32.3Mg2Si0.78AlSb1.6Ru2Ge30.34Mg2Ge0.70GaSb0.8Mg2Sn0.36ReSi20.12Mg2Pb0.10FeSi20.9BaSi20.48Ru2Si30.9MnSi1730.67Si1.1CrSi20.35Ge0.60SixGe1-x0.7Sn0.10
To recapitulate, most semiconductors, particularly those used in thermoelectric applications, normally have low to moderate energy band gaps, and are selected or produced, so as to have high atomic masses, in order to lower the thermal conductivity. Many semiconductors are either soft or brittle, have covalent chemical bonds, are somewhat chemically unstable, or reactive with atmospheric oxygen and moisture, and have low to moderate melting points.
In 1956, A. F. Ioffe conceived of the idea of alloying, or forming solid solutions of, isomorphic semiconducting compounds, in order to lower the thermal conductivity of thermoelectric materials. The foregoing is due to phonon-phonon interaction, and the resulting phonon-phonon scattering, the rate of which increases with increasing temperature, simply because there are more phonons around. In the quantum mechanical picture of phonons, this type of phonon-phonon scattering is described as the absorption, or emission, of one phonon by another phonon. Thus, in phonon-phonon interaction, the incident or incoming phonon increases in energy due to its interaction with the obstacle, and the absorption of one phonon. Phonon emission is similar except that the incident or incoming phonon loses energy, and the obstacle is represented by an emitted phonon.
The next most important source of scattering for phonons is due to point defects. A point defect simply means that one of the atoms making up the crystal is different from all of the others. A point defect is, by definition, very small, and has little or no effect on long wavelength or low energy phonons. But short wavelength, high energy, phonons are strongly scattered by point defects. Any type of defect will scatter phonons, but the most important type of point defect in thermoelectric materials is usually an atom with a mass very different from that of the host.
When the main difference between the point defect and the host is the mass of the atom, the scattering is often called “alloy scattering,” “mass fluctuation scattering,” or “mass fluctuation alloy scattering.” By the same token, when the main difference between the point defect and the host is the volume of the atom, the scattering is called “volume fluctuation scattering,” or “volume fluctuation alloy scattering.” Normally, the main difference between the point defect and the host involves both the mass and volume of the atom. Thus, both mass fluctuation scattering, and volume fluctuation scattering, usually take place simultaneously. Consequently, the term “alloy scattering” generically implies point defect phonon-phonon scattering, due to both mass and volume fluctuations, or differences, between the point defects and the host atoms. The terms: “mass and volume fluctuation scattering” or “alloy scattering” are generally preferred over the term; “point defect scattering,” when the point defect atoms are present in quite substantial proportions in the mixture, or alloy, composed of both the defect and host atoms. But the idea, or principle, remains the same: if the crystal lattice is really uniform, phonons travel with very little scattering. Whereas, when the lattice has lots of defects, phonons are strongly scattered.